INTRODUCTION

This mod lets all campfires illuminate the surrounding terrain which is turned off for some campfires and I have no idea why. 
The mod doesn't add any lights so it shouldn't cause the light flickering bug.

After combing through all the worldspaces and especially all the "Wilderness" cells, this mod is now officially finished.

I "fixed" all campfires in the Tamriel Worldspace located in these cells:
- BrucasLeapRedoubtExterior
- CradlecrushRockExterior
- DeepwoodRedoubtExterior02
- DragonBridgeExterior03
- DushnikhYalExterior02
- HagRockRedoubtExterior03
- IronBindBarrowExterior01
- KarthspireRedoubtExterior (2 fires)
- KorvanjundExterior03
- LargashburExterior03
- LostValleyRedoubt03
- LostValleyRedoubt04
- MarkarthAttackStart02
- MilitaryCampFalkreathImperial
- MilitaryCampHaafingarSons
- MilitaryCampHjaalmarchImperial
- MilitaryCampHjaalmarchSons
- MilitaryCampPaleImperial
- MilitaryCampPaleSons
- MilitaryCampReachSons01DUPLICATE000
- MilitaryCampRiftImperial
- MilitaryCampRiftSons
- MilitaryCampWhiterunImperial
- MilitaryCampWhiterunSons
- MilitaryCampWinterholdImperial
- MilitaryCampWinterholdSons
- OrphanRockExterior02
- RedEagleRedoubtExterior01
- RedEagleRedoubtExterior03
- RiftenAttackStart01 (2 fires)
- SerpentsBluffRedoubtExterior02
- SolitudeAttackStart01
- SolitudeAttackStart03
- StonehillsExterior01
- TalkingStoneCampExterior02
- WhiterunAttackStart03 (2 fires)
- WhiterunExterior14
- WindhelmAttackStart01
- WindhelmAttackStart03

There is one campfire where I had second thoughts about fixing it so I made an optional .esp which includes that campfire.
You can look at the screenshots I added for comparison: Left is without the fix and bottom is with Vivid Weathers active.

- Optional campfire in the DeepwoodRedoubtWorld Worldspace in the cell Wilderness [-31, 18]

Feel free to let me know if I missed anything and help testing to make sure that unchecking this box in the CK does not have any nasty side effects.
If it doesn't have side effects, then why did Bethesda check those boxes?

INSTALLATION

Unzip and put the .esp in your data folder
--or--
Use a mod manager tool like Vortex

AUTHOR'S NOTES

Thank you guys for the nice comments and endorsments, they really make my day!
To celebrate 100 endorsements, the latest version now comes with a fomod installer!

For the weirdo that is going to check my .esp to see if I unchecked this box on "all" campfire lights: Some campfires are placed on a tower for example, with no terrain near them meaning that unchecking the box for these campfires has no visual improvement.
(a tower is an object and not a terrain so it is lit even when the box is checked)
==> I don't touch what doesn't need touching.

There are also a few campfires that are embedded in a structure which I didn't fix because it made the terrain outside of the structure light up in an unrealistic way. Such structures are lit up anyway so fixing those campfires would have little to no improvement along with that huge drawback.

I am building a vanilla mod setup which is meant to only have mods that improve upon the vanilla game but still retain the vanilla feel as close as possible. 
Lighting mods are super awesome and offer a great global improvement but they often come with drawbacks in the form of flickering lights in certain areas when you have multiple mods installed.
Lighting mods also change lighting placements which is probably for the better but can take away the vanilla feel or cause conflicts in certain areas.
These are the reasons why I don't include lighting mods in my vanilla mod setup.
Playing with this setup (aka w/o  lighting mods), I came across this campfire in Dragonbridge which looked really out of place. 
After a little testing I found out that a little checkbox in the CK caused this "problem" and unchecking it had 0 drawbacks: no light flickering or anything.
This mod is meant to fix these little annoyances and improve upon the vanilla game, not add anything else.